1. Field of the Invention
The invention relates to a device and method for stabilising a Fabry-Perot interferometer including two plane mirrors having optical surfaces arranged parallel to one another with a preselected optical distance between the optical surfaces of the mirrors.
2. Prior Art
The Fabry-Perot interferometer is a very high resolution spectrometer commonly used for analysing visible light. As shown in FIG. 1a it consists of two very flat mirrors 1 and 2 arranged accurately parallel to one another with a suitable scanning device 10a and 10b (such as a piezoelectric translator) which enables the spacing L between the mirrors to be varied.
The device acts as a tuneable resonator. Light incident perpendicularly on the first mirror will be transmitted by the interferometer whenever the wavelength λ satisfies the condition2nL=pλ  (1)where L is the spacing between the mirrors, n is the refractive index of the medium between the mirrors and p is an integer. In many applications the spacing L is varied by piezoelectric means.
The transmission curve for the interferometer is given byT=1/[1+(4F2/π2)sin2(nL·2π/λ)],  (2)and shown in FIG. 1b as a function of mirror spacing L.
Because the light makes many reflections between the mirrors the resonant peaks are sharp. The ratio of peak spacing to peak width is known as the finesse F. The finesse depends on mirror flatness and reflectivity and in typical applications values of finesse of around 30–100 are used.
Although the Fabry-perot is a very useful instrument in view of its very high resolution, it is a highly sensitive device which is difficult to keep stable. Referring to FIG. 1b it is seen that a change in mirror spacing of only 3 nm is needed to scan through the transmission peak. For stable operation therefore both mirror spacing and parallelness must be maintained to an accuracy of the order of 1 nm.
In practice such a high stability is very difficult to achieve. Even using low expansion materials for the construction, purely passive stability would require maintaining a temperature stability of better than 0.1° C. and even then mechanical relaxation tends to limit the performance.
Although active feedback stabilisation is often used in practice, this can only be used in a scanning mode and normally requires a reference beam. The feedback system works by modulating the mirror spacing and alignment in order to maximise the height of the transmitted reference beam. Such systems are complex and cannot be used to stabilise a non-scanning interferometer.
A scheme using white light fringes has been used for maintaining parallelism even in a non-scanning interferometer but the scheme cannot be used for maintaining mirror spacing.